Unit 3 (Part-I): Greedy Algorithms

Transcription

1 Unit 3 (Part-I): Greedy Algorithms

2 Expected Outcomes of topic (CO) Understand problem and its formulation to design an algorithm Demonstrate the knowledge of basic data structures and their implementation and decide upon use of particular data structure best suited for implementing solution. Write efficient algorithms using Greedy, Divide & Conquer to solve the real life problems. Trace logic formulation, execution path of particular algorithm and data generated during execution of algorithm.

3 Basic Characteristics Greedy algorithms

4 Greedy Algorithms Characteristics Greedy Algorithms are short sighted. Greedy Algorithms are most efficient if it works. For every instance of input Greedy Algorithms makes a decision and continues to process further set of input. The other input values at the instance of decision are lost and not used in further processing. 5 A 8 A 5 8 S D S D t 9 B 2 y x

5 Greedy Algorithm: Knapsack Problem Principle: Knapsack is a bag of fixed capacity. In this problem it is assumed that n objects with profit value and weight are given. The main objective is to select the objects and place them in knapsack such that the object placed in the bag will generate maximum profit. Two types of Knapsack problems: Fractional Knapsack problem 0/1 Knapsack problem The Fractional Knapsack problem can be solved using Greedy approach, where as 0/1 Knapsack problem does not have greedy solution.

6 Representation Capacity = 20 10/2 9/4 15/3 6/6 4/1 11/5

7 Greedy Algorithm: Knapsack Problem Approaches: There are three basic approaches to solve knapsack problem: Select the maximum profit objects Select the minimum weight objects Select the objects based on Profit/Weight ratio

8 Significance TIME PROFIT Process for execution Generate maximum Profit in available time by proper selection of processes

9 Greedy Algorithm: Knapsack Problem Example 1: Capacity = 15, number of objects = 7 Profit Weight Index Approach: Maximum Profit Object Weight Profit Capacity Remaining Partial/Complete O =11 C O =6 C O =4 C O4* =0 P TOTAL

10 Greedy Algorithm: Knapsack Problem Example 1: Capacity = 15, number of objects = 7 Profit Weight Index Approach: Minimum Weight Object Weight Profit Capacity Remaining Partial/Complete O =14 C O =13 C O =11 C O =8 C O =4 C O3* 5(4) =0 P TOTAL

11 Greedy Algorithm: Knapsack Problem Example 1: Capacity = 15, number of objects = 7 Profit Weight Ratio Index Approach: Maximum Pi/Wi ratio Object Weight Profit Capacity Remaining Partial/Complete O =14 C O =12 C O =8 C O =3 C O =2 C O2* 3(2) 10/3= =0 P TOTAL

12 Greedy Algorithm: Knapsack Problem Example 2: Capacity = 18, number of objects = 7 Pi Wi Ratio Index Maximum (Pi/Wi) Ratio Object Weight Profit Capacity Remaining Partial/Complete

13 Exam Questions

14 Knapsack Problem: Algorithm Data Structures: Weight array: W[n] To store weight values Profit array: P[n] To store profit values Capacity Given and will decrease after each step X[n] To store 0, 1 or fraction depending upon object placement Weight = 0 (initially) and will increase towards capacity Profit = 0 and will be calculated by X[i] and P[i] Process: To select the object with best Pi/Wi Ratio. (Sorting)

15 Knapsack Algorithm Initialization steps: Profit Weight x[1..n] to hold data about object placement [partly, completely, rejected] Decision step While ( condition ) Select object //Call function for selecting best suited object// Place in bag {Completely / Partially } Update x [i] Final step: Find profit Multiply x[i] and associated profit

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17 Greedy Algorithms on Graph Prim s Algorithm Reverse Delete Algorithm

18 Minimum Cost Spanning Tree Given a graph G =(V,E) V=Number of vertices and E=Number of edges, then Spanning tree is tree generated from graph, with characteristics All the vertices present in the tree There is no cycle in the tree i.e., e=v-1 Subset of edges, that forms a tree, where total cost of edges is minimum. Minimum cost spanning tree is a spanning tree with edges of minimum cost. Applications: To transmit the message without broadcasting. To generate travel plan in minimum cost. Designing network, home electric wiring etc. Approaches: Prims Method and Kruskals Method.

19 Minimum Cost Spanning Tree Principle: Select an edge of minimum cost. The edge will derive two vertices. Continue the process from one of the vertex by selecting the next edge of minimum cost. Once the vertex is visited, then it is marked. The process will attempt to visit unmarked vertices and terminates when all the vertices are visited. Process guarantees no cycle in the tree. What if the process of tree generation starts from any arbitrary vertex. Motivation: To join points as cheaply as possible: Applications in clustering and networking Acyclic graph to connect all nodes in minimum cost. One of the term in U.S. legal code (AT&T)

20 Graphs for Examples Steiner Spanning Tree

21 Spanning tree: Free tree A free tree has following properties 1. Exactly n-1 edges for n vertices 2. There exists a unique path between two vertices. 3. By adding an edge, a cycle will be created in free tree. Breaking any edge on the cycle restores the free tree. Greedy: Minimization problem. 1. Repeated selection: of minimum cost edges with certain test cases. 2. Once decision of adding edge is finalized, it cannot be revoked. 3. Basic idea: To generate subset of E connecting all V

22 Minimum Cost Spanning Tree: Prim s method Example: Consider the following graph: Select an edge of minimum cost. From selected vertex continue selecting edge of minimum cost

23 Minimum Cost Spanning Tree: Prim s method Example: Consider the following graph: The next vertex should be the vertex reachable in minimum 5 45 cost either 40 from 35 vertex 99 1 or vertex Out of Four possible options: cost[near[j],j] = minimum Select the index satisfying above condition: T C F R < 3 2 F 4 1 < 4 2 T 5 1 < 5 2 F 6 1 < 6 2 F

24 Minimum Cost Spanning Tree: Prim s method The vertex selected is 6 due to which there may be changes in closeness of vertices. That the Example: vertices which were close to vertex 1 or vertex 2 may modified due to selection of vertex 6. Consider the following graph: Out of 2 Four possible 3 options: 4 5 cost[near[j],j] 6 = minimum - Select - the index 2 satisfying 1 above 2 condition: T C F R < 3 2 T 4 1 < 4 6 F 5 6 < 5 2 F 6-6

25 Minimum Cost Spanning Tree: Prim s method The vertex selected is 3 due to which there may be changes in closeness of vertices. That the Example: vertices which were close to vertex 1 or vertex 2 or vertex 6 may modified due to selection of vertex 3. Consider the following graph: Out of 2 Four possible 3 options: 4 5 cost[near[j],j] 6 = minimum - Select - the index 2 satisfying 1 above 2 condition: T C F R < < 4 6 F < 5 2 T 6-6

26 Minimum Cost Spanning Tree: Prim s method The vertex selected is 4 due to which there may be changes in closeness of vertices. That the Example: vertices which were close to vertex 1,2,6,3 may modified due to selection of vertex 4. Consider the following graph: T C F R < 3 4 < 4 F < 5 4 T 6-6

27 Prim s method: Output Example: Consider the following graph: Vertex 1 Vertex 2 Cost TOTAL 105

28 Algorithm: Prim s Algorithm Data Structures: Input cost[1..n, 1..n] To store the cost information Intermediate: near[1..n] To store near information for selected vertex Output: mincost=0 t[1..n-1, 1..3] To store the spanning tree

29 Algorithm Algorithm Prim(cost,n: mincost,t) { Step 1: Select an edge of minimum cost from the cost matrix and let it be cost[k,l] mincost = cost[k,l]; t[1,1] = k; t[1,2] = l; t[1,3] = cost[k,l] for 1 = 1 to n do { if cost[i,l] < cost[i,k] then near[i] = l; else near[i] = k; } //end of for for i = 2 to n-1 do { let j be an index such that (near*j+ 0 and cost*near*j+, j+ = minimum) t[i,1] = j; t[i.2] = near[j] t[i,3] = cost[near[j],j] near[j] = 0; for k = 1 to n do if((near*k+ 0) and cost[k,near[k] > cost[k,j])) near[k] = j; } //End of for } //End of algorithm

30 Minimum Cost Spanning Tree: Prim s Algorithm Designed by Robert C. Prim in 1957 and was modified by Dijkstra so also called as DJP algorithm.

31 Proof of Correctness: Prims Algorithm The proof of prims algorithms is described using CUT property of a graph. Let G=(V,E) be the graph, then CUT property will divide the graph into two sets of vertices (X and V-X), such that there exists three types of edges. Edges with both vertices in X Edges with both vertices in V-X Edges with one vertex in X and one vertex in V-X Crossing edge If there are n vertices in graph, then number of cuts will be 2 n Empty cut lemma: A graph is said to be not connected, if cut has no crossing edges. This is denoted as empty cut. The correctness is proved using straight forward induction method.

32 Reverse Delete Algorithm: application of DFS

33 Algorithm: RD method EDGE CG cannot be deleted, as there is no Alternative path. If from vertex C or Vertex G, DFS is performed It is not possible to reach to other vertex.

34 Practice examples: Prims & RD algorithm

35 Solution